Experimental and Numerical Study for Improved Understanding of Mixed-Convection Type of Flows in Turbine Casing Cavities During Shut-Down Regimes

Abstract

Due to increase in the power generation from renewable sources, steam and gas turbines will be required to adapt for more flexible operations with frequent start-ups and shut-downs to provide load levelling capacity. During shut-down regimes, mixed convection takes place with natural convection dominance depending on the operating conditions in turbine cavities. Buoyant flows inside the turbine that are responsible for non-uniform cooling leading to thermal stresses and compromise clearances directly limits the operational flexibility. Computational fluid dynamics (CFD) tools are required to predict the flow field during these regimes since direct measurements are extremely difficult to conduct due to the harsh operating conditions. Natural convection with the presence of cross-flow -mixed convection has not been extensively studied to provide detailed measurements. Since the literature lacks of research on such flows with real engine representative operating conditions for CFD validation, the confidence in numerical predictions is rather inadequate. This paper presents a novel experimental facility that has been designed and commissioned to perform very accurate unsteady temperature and flow field measurements in a simplified turbine casing geometry. The facility is capable of reproducing a wide range of Richardson, Grashof and Reynolds numbers which are representative of engine realistic operating conditions. In addition, high fidelity, wall resolved LES with dynamic Smagorinsky subgrid scale model has been performed. The flow field as well as heat transfer characteristics have been accurately captured with LES. Lastly, inadequacy of RANS for mixed type of flows has been highlighted.

Experimental and numerical investigations of mixed convection in turbine cavities for more flexible operations

“…In Case-2, on the other hand, the thermal plume is not formed due to the strong cross flow, therefore leading to more uniform thermal boundary layer thickness around the inner cylinder than Case-1. The lower portion of the annulus in both cases resembles each other with thin laminar boundary layer as also reported in the study of Murat et al (2021). Time-averaged non-dimensional temperature profiles predicted by LES are compared against the experimental data in Figure 14 for Case-1 at Section 1, 2 and 3.…”

“…Measurement locations on the test facility are depicted in the facility schematic in Figure 3 and some of the essential dimensions are tabulated in Table 1. The detailed design information regarding the experimental facility can be found in the study of Murat et al (2021).…”

“…The experimental investigations concerning the mixed type type of flows have been mostly on fully developed duct and pipe flows (for example Kotake and Hattori (1985), Ciampi et al (1987) and Mohammed et al (2010)). In the recently conducted research of Murat et al (2021), a unique experimental facility has been designed and commissioned for mixed type of flows in turbine casing cavities during shut-down regimes, and also incapabilities of Reynolds-averaged Navier-Stokes (RANS) in predicting these type of flows have been highlighted by demonstrating the inadequacy of isotropic turbulence assumption.…”

Experimental and Numerical Investigations of Mixed Convection in Turbine Cavities for More Flexible Operations

“…3 and some of the essential dimensions are tabulated in Table 1. The detailed design information regarding the experimental facility can be found in the study of Murat et al (2021). For pure natural and forced convective flows, Grashof and Reynolds numbers are important non-dimensional parameters to determine the flow regime.…”

“…The experimental investigations concerning the mixed type type of flows have been mostly on fully developed duct and pipe flows (for example Kotake and Hattori (1985); Ciampi et al (1987) ; Mohammed et al (2010)). In the recently conducted research of Murat et al (2021), a unique experimental facility has been designed and commissioned for mixed type of flows in turbine casing cavities during shut-down regimes, and also incapabilities of Reynolds-averaged Navier-Stokes (RANS) in predicting these type of flows have been highlighted by demonstrating the inadequacy of isotropic turbulence assumption.…”